In recent years, mobile radio telecommunication systems have widely spread. Such mobile radio telecommunication systems (for example GSM, 3rd generation system such as the Universal Mobile Telecommunications System (UMTS) and others) operate with different data transmission methods. Such a data transmission method is for example a Code Division Multiple Access (CDMA) method. Said CDMA method is further developed into a Wideband Code Division Multiple Access (WCDMA) method (or Broadband CDMA) using a greater frequency band for example to be used in 3rd generation communication systems like the UMTS.
In both CDMA and WCDMA methods, the basic operating principle is similar. Data to be transmitted between terminal devices such as mobile stations via a communication network comprising several network elements such as transceiver devices such as base transceiver stations, mobile switching centers and the like is multiplied with a unique code allocated to each connection. Said code has a higher frequency than the data which results in a wide transmission bandwidth in comparison with the original data bandwidth. This process is also known as spreading.
On a receiving side knowing the respective code, the transmitted signal is decoded and the recovered data is processed further. This process is also known as despreading.
For a correct recovery of the data from the transmitted signal a major condition is that the received signals have a (nearly) constant and equal strength. Since in a mobile telecommunication system, for example due to a movement of the terminal devices, widely different signal strengths may be received when transmitting always with the same transmission power, interference between different terminal devices transmitting at the same time may occur. Therefore, an accurate power control in uplink (terminal device to transceiver station) and downlink (transceiver station to terminal device) is required.
In third generation CDMA systems (e.g. cdma2000, WCDMA) fast power control is used both in uplink and downlink. Here, the decoding performance is optimal when the received signal power is as constant as possible. This is required especially when the service delay is limited (for example in speech communication) and a fading may cause errors. The fast power control is able to follow even fast fadings.
On the other hand, the downlink transmission capacity is maximized when the transmitted power is minimized, i.e. the generated interference is as low as possible. However, fast power control aims that the received power is constant. This means that the transmitted power varies greatly, which increases the transmitted power average. Also, the variation of transmitted power causes power spikes which are harmful for system load control.
However, in case of e.g. a packet data service, a retransmission protocol is included. This means that a loss of data, for example due to decoding problems, would not cause absolute errors but only lead to a retransmission. This allows a packet data power control not to follow every fade, but to compensate fades by retransmissions.
When using a slower power control occurrence of high interference spikes as in the fast power control can be prevented. However, slower power control works poorly for delay limited services as it can not follow fast fading of the signal.
In document EP-A-0 682 417, a transmission power control method of a spread spectrum communication system is disclosed which determines transmission power in accordance with a transmission power control (TPC) bit.
In document EP-A-0 682 419, a transmission power control method is disclosed which uses an open or a closed loop control in dependence on changes in the desired received signal level at the mobile station.